60-65mm ID spring sitting on rear lower control arm

[luckyu]

I'm changing to adjustable springs in the rear, using 60mm or 65mm ID flat-ended springs.

What is used to mate the flat-bottomed spring to the lower control arm. Is the stock M3 spring seat reused?

I also need to choose a height-adjuster. It looks like there are choices from HP Autowerks (60mm), TC Kline (60mm), and AST (2.5"). H&R may make one too, but I can't find it sold separately.

EDIT: I found H&R's height adjuster, it takes 60mm springs and is sold by Turner Motorsport.

[JAJ]

Originally Posted by luckyu

I'm changing to adjustable springs in the rear, using 60mm or 65mm ID flat-ended springs.

What is used to mate the flat-bottomed spring to the lower control arm. Is the stock M3 spring seat reused?

I also need to choose a height-adjuster. It looks like there are choices from HP Autowerks (60mm), TC Kline (60mm), and AST (2.5"). H&R may make one too, but I can't find it sold separately.

Ground Control makes a 2.5" unit as well.

[HP Autosport]

You will want to use 60mm springs as they can be located better by the M3 lower camber links. Don't forget to use Swift thrust sheets throughout to spread the spring load and to help dampen spring noise.

Our adjuster has the longer should if you should decide to run a thrust bearing to ease height adjustments. The adjuster's threaded portion is also made longer to accommodate a wider range of height adjustments.

[Richard@M-World]

The stock lower mount does not accept the flat ends of linear race springs, so it will need to be removed.

KW offers a lower spring seat for 60 mm springs to properly seat on the lower camber arm.

We also offer adjustable height adjusters from KW's Competition line.

May I ask why you are changing to adjustable rear?

[luckyu]

Originally Posted by JAJ

Ground Control makes a 2.5" unit as well.

Thanks, forgot about them. Also KW offers one.

[luckyu]

Thanks Harold, I will use 60mm then.

[luckyu]

Originally Posted by Richard@M-World

The stock lower mount does not accept the flat ends of linear race springs, so it will need to be removed.

KW offers a lower spring seat for 60 mm springs to properly seat on the lower camber arm.

We also offer adjustable height adjusters from KW's Competition line.

May I ask why you are changing to adjustable rear?

Thanks Richard. I'm installing adjustable rear springs because of an M3 camber link conversion on a 328i. I'll change my signature.

I was advised that my E9X springs won't fit, and I need to gain ride height. Plus, with adjustables I can choose the spring rate I need in the rear (500 lb/in) to match OE front and provide near-M3 frequencies).

[luckyu]

In an attempt to estimate the spring length required, I was able to measure, somewhat inaccurately, the length of the rear spring on the car. I get 8" to 8.5". To get it up to desired ride height, 14", it would need to be 8.5" to 9".

EDIT: Raising my 13.5" ride height 0.5" to 14" ride height only requires ~.28" at the spring.

Rate of new spring will be 504 lbs/in, wheel rate, 158 lbs/in. Desired ride height, 14". Load on the spring is estimated at 740 lbs.

My calculator says the new spring will compress 4.7" when the car rests on the ground.

EDIT: 4.7" at the WHEEL, which needs to be translated back to the spring by multiplying by 0.56. 2.6" inches of compression at the spring.

So I take 9", add 4.7", subtract 2" for a height adjuster, subtract 0.5" for seats, and I get 11.2".

11 inches seems like a longer spring than many are using. Most are using 9"?

EDIT: Once I do the math right, I get 9.1" which agrees with what's shipped out in most kits.

[luckyu]

By the way, if anyone happens to have a stock rear spring lying around, it would really help me if I knew how long it was. A stock sports package spring, or stock M3 spring. Thanks!

[Richard@M-World]

Stock M3 spring top to bottom: 11.75"

Keep in mind that the stock spring has a certain amount of pre-load on them when installed.

[HP Autosport]

Originally Posted by luckyu

In an attempt to estimate the spring length required, I was able to measure, somewhat inaccurately, the length of the rear spring on the car. I get 8" to 8.5". To get it up to desired ride height, 14", it would need to be 8.5" to 9".

Rate of new spring will be 504 lbs/in, wheel rate, 158 lbs/in. Desired ride height, 14". Load on the spring is estimated at 740 lbs.

My calculator says the new spring will compress 4.7" when the car rests on the ground.

So I take 9", add 4.7", subtract 2" for a height adjuster, subtract 0.5" for seats, and I get 11.2".

11 inches seems like a longer spring than many are using. Most are using 9"?

Depends on which suspension system you are looking at. All of our Koni, AST and KW Swift conversions are using 9" rears.

TCKline uses 10" rears.

[luckyu]

Originally Posted by Richard@M-World

Stock M3 spring top to bottom: 11.75"

Keep in mind that the stock spring has a certain amount of pre-load on them when installed.

Hey! Thanks! OK, here's a formula for estimating the length L of a spring necessary to drop an M3 by D inches.

A = spring height adjuster offset, in inches
D = amount you want to lower the M3, in inches
S = spring rate of your spring, in lbs/in

EDIT: There was an error in these formulas, I neglected to translate wheel compression back to spring.

L = 9.00" - A - 0.56*D + 0.56*(858lbs / 0.3136*S)

A more general formula, for understanding:

C = weight of rear corner of M3
x = 9.00" = loaded length of M3 spring sitting in M3 at rest
S = spring rate of your spring

L = x - A - 0.56*(D + C / (0.3136*S)
x = 11.75 - 0.56 * C / (0.3136*550)

Explanation:

D*0.56 is the amount you are lowering the loaded-spring height
The compression amount plus loaded spring length equals unloaded spring length.
x - A - D*0.56 is the loaded length of your spring in the car
C / 0.3136*S is the inches your spring compresses as car is brought to ground (measured at the wheel)

Sources of error in calculation:

- me
- me again
- assuming M3 spring is linear
- measured motion ratio of rear spring, 0.56
- estimated rear corner weight of M3, 858 lbs
- rear spring rate of M3, 550 lbs/in
- measured length of M3 spring, 11.75"

Additional things to check beside ride height when choosing a spring:

- Make sure the spring is long enough that it won't fall out at full droop.
- Check bump travel available before coil bind. Swift publishes stroke for all its race springs.
- Make sure bump stop interferes before coil bind occurs.
- If you do use a helper, when compressed it takes up space, affecting coil bind danger for the main spring.
Thanks JAJ!

[luckyu]

If I use this formula for my spring rate of 504 lbs/in, I get 10.43 inches.

EDIT: Again, my calculations here are wrong because the formula was wrong.

Now, that's to maintain the stock M3 ride height in an M3.

If we knew that the fenders of M3 and E9X are at the same height relative to the upper spring mount, I think I could translate between the two types of car.

It seems like more trouble than it's worth, doesn't it? But this is very valuable to me.

EDIT: That result, 10.43" is with an adjuster offset of 1.75".

[luckyu]

Here's a simpler version of the formula. To get stock ride height in an M3, with no adjuster and no spacers, take the Springrate (in lbs/inch) of your spring to calculate its Freelength (in inches):

EDIT: I've gotta stop making formulas so I don't have to go fix them all... Don't use this formula, use the ones above.

Freelength = 6.75" + 2736.0 / Springrate

After calculating Freelength, to add an adjuster, just subtract the adjuster offset (in inches) from the free length.

To drop the M3 by D inches from stock height, subtract D*0.56 from the free length.

If 2736 / Springrate is less than droop travel (which I believe is 4"), then you need a helper spring.

RequiredStroke = 2736 / Springrate + BumpTravel*0.69

BumpTravel is measured in inches at the shock. It is the amount of bump travel allowed by the bump stop.

If RequiredStroke > RatedMaxStroke of spring (from Swift spring chart), then you risk coil bind.

The M3 rear spring is probably slightly progressive, which means that the 6.75 number is a little low. Expect the real number -- the compressed length of an M3 spring sitting in an M3 at rest on the ground -- to be a smidge higher.

Another way to think of the 6.75 number is it's the amount of space you want the loaded spring to take up with car sitting on the ground. This helps visualize accounting for adjusters and desired drop.

I'm assuming the stock spring seat is used in the camber link. Many people don't do this, so in that case you have to add its contribution to the free length to account for its removal.

[JAJ]

These equations are probably correct, but while it gives you an answer, it won't necessarily give you a solution that works. The missing element is suspension travel.

Most race springs are, more or less, half iron and half air. For instance, an 8" spring has about 4" of travel from unloaded to coil-bound, maybe a touch more, but not a lot. When you use one of these formulas, make sure that you don't run out of spring travel before you get to the bump stop, or you risk damaging the suspension on a hard bump.

[luckyu]

Originally Posted by JAJ

These equations are probably correct, but while it gives you an answer, it won't necessarily give you a solution that works. The missing element is suspension travel.

Most race springs are, more or less, half iron and half air. For instance, an 8" spring has about 4" of travel from unloaded to coil-bound, maybe a touch more, but not a lot. When you use one of these formulas, make sure that you don't run out of spring travel before you get to the bump stop, or you risk damaging the suspension on a hard bump.

Thanks JAJ, you're right I had not thought about coil bind. I will try to work that in, or make it a caveat or separate calculation. Once I do that I'll edit the two "formula posts".

[luckyu]

I fixed an error in the formulas, 0.3136*D should've been 0.56*D.

I also added a formula for coil bind danger detection.

When you move the spring height adjuster, you get 1.78X the effect at the wheel. Move the adjuster 1/4", and you get almost half an inch change in ride height.

[luckyu]

I fixed another error in the formulas. Spring lengths were off by 2 inches.

With the corrections, 9" spring lengths are appropriate in most cases with ride heights this side of sanity. If you're going really low and hard, 8" can work.

This means that 60mm is the way to go for the rear spring, regardless of ride height. There is plenty of adjustability once the spring fits. A half inch of adjustment gives you almost an inch to play with at the wheel.

All reasonable choices for the rear spring are available in Swift 60mm 9".

Finally, most reasonable choices have more than 2 inches of linear bump travel.